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arXiv:quant-ph/0603112

Capacity regions for multiparty quantum channels

Maciej Demianowicz, Paweł Horodecki

Published Mar 13, 2006• 2 citations
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Abstract

We show equivalence of different capacity notions in case of multipartite communication and discuss some of its consequences. We also provide quantum capacity region for a quantum channel with $k$ senders and $m$ receivers in the scenario in which receivers are communicated by single senders. We also point out natural generalizations to the case of two-way capacity regions.

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We show equivalence of different capacity notions in case of multipartite communication and discuss some of its consequences. We also provide quantum capacity region for a quantum channel with kkk senders and mmm receivers in the scenario in which receivers are communicated by single senders. We also point out natural generalizations to the case of two-way capacity regions.
We analyze different aspects of multiparty communication over quantum memoryless channels and generalize some of the key results known from bipartite channels to the multiparty scenario. In particular, we introduce multiparty versions of subspace and entanglement transmission fidelities. We also provide alternative, local, versions of fidelities and show their equivalence to the global ones in context of capacity regions defined. An equivalence of two different capacity notions with respect to two types of fidelities is proven. In analogy to the bipartite case it is shown, via sufficiency of isometric encoding theorem, that additional classical forward side channel does not increase capacity region of any quantum channel with kkk senders and mmm receivers which represents a compact unit of general quantum networks theory. The result proves that recently provided capacity region of a multiple access channel [M. Horodecki et al., Nature 436, 673 (2005); J. Yard et al., e-print quant-ph/0501045], is optimal also in a scenario of an additional support of forward classical communication.
We consider quantum channels with two senders and one receiver. For an arbitrary such channel, we give multiletter characterizations of two different two-dimensional capacity regions. The first region comprises the rates at which it is possible for one sender to send classical information, while the other sends quantum information. The second region consists of the rates at which each sender can send quantum information. For each region, we give an example of a channel for which the corresponding region has a single-letter description. One of our examples relies on a new result proved here, perhaps of independent interest, stating that the coherent information over any degradable channel is concave in the input density operator. We conclude with connections to other work and a discussion on generalizations where each user simultaneously sends classical and quantum information.
We consider generalizations of the dense coding protocol with an arbitrary number of senders and either one or two receivers, sharing a multiparty quantum state, and using a noiseless channel. For the case of a single receiver, the capacity of such information transfer is found exactly. It is shown that the capacity is not enhanced by allowing the senders to perform joint operations. We provide a nontrivial upper bound on the capacity in the case of two receivers. We also give a classification of the set of all multiparty states in terms of their usefulness for dense coding. We provide examples for each of these classes, and discuss some of their properties.
We investigate the maximum rates for transmitting quantum information, distilling entanglement, and distributing secret keys between a sender and a receiver in a multipoint communication scenario, with the assistance of unlimited two-way classical communication involving all parties. First we consider the case where a sender communicates with an arbitrary number of receivers, a so-called quantum broadcast channel. Here we also provide a simple analysis in the bosonic setting where we consider quantum broadcasting through a sequence of beamsplitters. Then, we consider the opposite case where an arbitrary number of senders communicate with a single receiver, a so-called quantum multiple-access channel. Finally, we study the general case of all-in-all quantum communication where an arbitrary number of senders communicate with an arbitrary number of receivers. Since our bounds are formulated for quantum systems of arbitrary dimension, they can be applied to many different physical scenarios involving multipoint quantum communication.
The depolarizing channel is one of the basic quantum channels.For simplicity and generality, quantum channels with two senders and one receiver are considered to discuss the capacity of the quantum depolarizing channel.Two different capacity regions,classical-quantum region and quantum-quantum re- gion,are deduced in theory for the quantum depolarizing channels with two senders and one receiver.The first region is comprised of the rates at which it is achievable for one sender to transmit classical information, while the other to send quantum information.The other region consists of the rates at which each sender sends quantum information.At the same time,the performance of the quantum multiple access depolarizing channel is analyzed.